breadloaf/BreadedSNES
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Add some branching instructions

Browse Source
This commit is contained in:
Palindromic Bread Loaf
2025-07-22 11:31:10 -04:00
parent afa00385db
commit 268b45b062
2 changed files with 61 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files

52
src/cpu.cpp
View File

@@ -100,9 +100,33 @@ void CPU::UpdateCompareFlags16(const uint16_t reg_value, const uint16_t compare_
}
}
void CPU::DoBranch(const bool condition) {
int8_t displacement = static_cast<int8_t>(ReadByte(PC));
PC++;
if (condition) {
// Save the current PC for page boundary check
const uint32_t old_pc = PC;
PC = static_cast<uint32_t>(static_cast<int32_t>(PC) + displacement);
cycles++;
if ((old_pc & 0xFF00) != (PC & 0xFF00)) cycles++; // Add one cycle for crossing a page boundary
}
}
void CPU::ExecuteInstruction() {
// TODO: Actual Opcode decoding
switch (const uint8_t opcode = bus->Read(PC++)) {
// Branch Instructions
case 0xF0: BEQ_Relative(); break; // BEQ $nn
case 0xD0: BNE_Relative(); break; // BNE $nn
case 0x90: BCC_Relative(); break; // BCC $nn
case 0xB0: BCS_Relative(); break; // BCS $nn
case 0x30: BMI_Relative(); break; // BMI $nn
case 0x10: BPL_Relative(); break; // BPL $nn
// CMP - Compare Accumulator
case 0xC9: CMP_Immediate(); break; // CMP #$nn or #$nnnn
case 0xCD: CMP_Absolute(); break; // CMP $nnnn
@@ -116,12 +140,12 @@ void CPU::ExecuteInstruction() {
case 0xCF: CMP_Long(); break; // CMP $nnnnnn
case 0xDF: CMP_LongX(); break; // CMP $nnnnnn,X
// CPX - Compare X Register
// CPX - Compare X Register
case 0xE0: CPX_Immediate(); break; // CPX #$nn or #$nnnn
case 0xEC: CPX_Absolute(); break; // CPX $nnnn
case 0xE4: CPX_DirectPage(); break; // CPX $nn
// CPY - Compare Y Register
// CPY - Compare Y Register
case 0xC0: CPY_Immediate(); break; // CPY #$nn or #$nnnn
case 0xCC: CPY_Absolute(); break; // CPY $nnnn
case 0xC4: CPY_DirectPage(); break; // CPY $nn
@@ -1467,4 +1491,28 @@ void CPU::JMP_AbsoluteIndirectX() {
PC = (static_cast<uint32_t>(PB) << 16) | target_addr;
cycles += 6;
}
void CPU::BEQ_Relative() {
DoBranch(P & FLAG_Z);
}
void CPU::BNE_Relative() {
DoBranch(!(P & FLAG_Z));
}
void CPU::BCC_Relative() {
DoBranch(!(P & FLAG_C));
}
void CPU::BCS_Relative() {
DoBranch(P & FLAG_C);
}
void CPU::BMI_Relative() {
DoBranch(P & FLAG_N);
}
void CPU::BPL_Relative() {
DoBranch(!(P & FLAG_N));
}